Horizon (1964) s00e75 Episode Script

The Truth About Meteors - A Horizon Special

For the residents of the
Russian city of Chelyabinsk,
the morning of Friday,
February 15th, 2013
began like any other.
As they set off to work, in what
has become a craze throughout Russia,
many recorded their journeys.
But these cameras,
usually used for capturing
minor traffic incidents,
were about to record history.
A fireball brighter than the
sun appeared from nowhere
..before exploding with
the power of 30 Hiroshimas.
A minute later,
a shockwave blew in the windows of
4,000 buildings across the region.
The broken glass accounting
for most of the 1,200 injured.
The people of Chelyabinsk had
just experienced the most powerful
meteor strike for more than a century.
The meteor that exploded over
Chelyabinsk is a spectacular
reminder of just how
exposed our world is.
Earth is this tiny planet
in a vast, violent cosmos.
It is also a reminder of the
powerful impact that these
alien rocks can have on
the fate of our planet
and on us.
This is not the first
time it has happened.
Over the last few years,
scientists have examined many other
devastating impacts in the earth's past.
Using this knowledge, I want
to answer the key questions
that the Chelyabinsk
meteor strike raises.
Where did this alien rock come
from? When will the next one strike?
And can we do anything
to protect ourselves?
A fortnight after the impact,
the meteor strike is
still big news in Russia.
In Chelyabinsk there is a
popular new winter pastime -
hunting for any fragments
of the meteorite that remain.
Scientists have also been out in force,
particularly around Lake Cherbarkul
where there is evidence
of an impact in the ice.
So many fragments have been found here,
it has been called the
Cherbarkul meteorite.
They are trying to piece
together exactly what happened,
because the fact is no-one in the
scientific world saw this coming.
I was shocked. I was truly shocked.
I never thought I would see
an event like this over a major
city during my lifetime.
We could not predict
this was going to happen.
The piece of rock that entered the
atmosphere was relatively small,
maybe only a few metres
across, and so we could not see
this before it entered.
When something like this happens,
there is no doubt about it,
it is frightening.
But I have to admit, as a geologist
witnessing a
once-in-a-lifetime event,
it is utterly thrilling.
You only had to look at social
media to see that scientists
all over the UK and around
the world were getting very,
very excited about
this as the news broke.
It was exciting. It was exciting
for me as a meteoriticist
because you immediately
want to know, what is it?
What has landed? Is it a bit of
Mars or a bit from an asteroid?
I am almost ashamed that I had
such great excitement about seeing
this event and knowing
that meteorites had fallen,
because people had been injured.
Chebarkul was the biggest
meteorite to strike the Earth
since we've had the
technology to measure them.
From its journey through the atmosphere
to its spectacular end,
every moment was captured.
One of the best documented
16 seconds in science ever.
'Professor Alan Fitzsimmons
is one of the scientists'
who has been examining the
meteorite footage frame by frame.
These are amazing images,
but what can you get out
of these as an expert?
What it shows us, first
of all, is a great record
of the entry of the object
into the Earth's atmosphere,
so you see it right from the moment
it really penetrated and there it is.
- That is the edge of the atmosphere?
- That is it and it is coming down at a fairly shallow angle
probably and as we play the movie
on, what we see is, bang, there,
it suddenly got brighter. So
something has happened to the object.
It is starting to break apart and
as it breaks apart, it releases
some of its orbital energy and that
is causing that big flare up there.
Is that because the atmosphere
is denser, it is harder?
That's right, it is finding it harder and
harder to punch through the atmosphere.
As we roll on, we suddenly get
this bang, this huge flare up where
suddenly the whole object is
starting to fragment and break apart.
That is where the majority of
the energy is being released.
If we look here.
There are just little bits falling off.
There is another flare up here and
there is another flare up there,
and now we can still see
it's glowing, incandescent,
some major fragment of the object is still
falling down through the Earth's atmosphere.
Underneath that trajectory
you are going to have showers
of bits of asteroid,
essentially, falling down,
and then finally 16.5 seconds later,
- and what we are left with is this contour trail.
- The shockwave.
The shockwave coming
towards us, that is right.
It is about a minute later that
it has gone and reached the ground.
- This guy driving doesn't know yet that the shockwave is on its way here.
- That is right.
He is still happily listening
to the radio on his drive to
work in the morning.
The explosion generated a shockwave
so massive it was detected
over 15,000 kilometres away.
The low-frequency waves were
picked up by monitoring stations.
This is kind of like a listening
network around the world.
That is right. They're not set up
for fireball or asteroid impacts,
but set up to listen
for nuclear explosions.
What the monitoring stations
picked up were some of the largest
infrasonic waves ever recorded.
Here they have been modified
to make them audible.
It has been detected down in
Antarctica, we've got records of it
up there in Alaska, so the pressure
wave from the entry of the object
and the explosive fragmentation
was found, seen all over the world.
So from the data that is coming
in, it is early days, obviously,
but from the data that is coming in, what is
your best guess at the size of that rocky lump?
Well, from the infrasound
we know the energy released
was something like 500,000
kilotons of energy, which is huge.
- I was thinking it sounded a lot.
- That is right.
And because we know it came in,
from the video footage, at
about 17.5 kilometres per second,
we can combine that energy with that
velocity to get a mass of the object.
From that mass, we can get a size
and it is probably about
15 metres across or so.
That is a rarity, isn't it?
We think these things come in
maybe, in once every 50 or 100 years,
that is all randomly, so this is a really
special and really rare event of course.
Meteor strikes as big
as this may be rare
but scientists have a surprisingly
detailed knowledge of what
meteorites are and where they come from.
Long before the meteorite reached
its explosive finale in full
view of Chelyabinsk's dash cams,
it had a very different existence
and going by a very different name.
Meteorites begin life in deep space,
as part of much larger
bodies called asteroids.
These can range in size
from just a few metres to
more than 900 kilometres.
The leftovers from the nebula
that created our solar system
some 4.6 billion years ago.
And millions of them
circle the sun in a trail
known as the asteroid belt.
Here, collisions
create smaller fragments
and when these fall towards Earth,
they take on one of two forms.
The smallest pieces will burn up
in the atmosphere to become meteors,
what we call shooting stars.
Only the larger fragments that
make it all the way to the earth's
surface are called meteorites.
The meteorite is a
piece of rock from space,
or a piece of metal from space,
that falls through our atmosphere
and actually hits the
ground to be recovered.
Technically, scientists
love their words, it is
not a meteorite before it is
actually found and discovered.
By collecting and comparing meteorites,
scientists have been able to piece
together a picture of how they form
and these studies have revealed
some of the most remarkable rocks
in the solar system.
Few places in the world have got as many
meteorites as the Natural History
Museum, meteorites like this.
It is a cracker, isn't it?
The ones that are out here on
display are just a fraction.
The bulk of the collection
is behind the scenes
and that is where the science goes on.
So all these are meteorites
in some shape or form?
They are all either meteorites.
'Professor Sara Russell is expert
at decoding the messages hidden
'within these fragments of space rock.'
This looks quite rocky,
what about that one?
It looks like a humble
rock but if you hold it -
be very careful of this one, this
is older than the Earth, it is
the oldest thing you will ever hold.
- This is older than what, 4.6 billion years?
- Yes.
We have this number of 4.6 billion
years of the age of the solar system,
we know that from
meteorites like this one,
and from looking at the age
of the components within it.
If you can see, it has these rounded
objects in it, which are 1mm to 1cm
in size, these are called chondrules,
and these were once free-floating.
Before there were planets, these
were free-floating in the solar system
around the very young sun
and then they slowly coalesced
to make asteroids and
larger and larger objects,
until eventually planets were formed.
These were the building
blocks of planets.
So the Russian meteorite,
any news on what kind it is?
Well, the early reports are
that it is an ordinary chondrite
and that means it will be similar to
this one, so this is really exciting
for us as scientists because we want
to know how the planet is formed,
what was around before the planets,
what the environment was like
and how the material that made up
the planets first came together,
and the chondrites are the
best way of finding that out.
Is this the most common
in the solar system?
It is the most common
type to fall down to Earth.
There is almost certainly a bias
that the only material that we get
to Earth is stuff that happens
to cross the Earth's orbit.
It has to be going in a slightly
odd direction to cross the Earth
anyway, so there is some
kind of selection bias.
This is a really special thing for
you to kind of have in your career.
Yes, if only something like
this would happen in Britain
so we could go and get it.
I don't think there's too many
people watching this programme
that will be saying, "I
wish it happened in the UK."
- Obviously somewhere uninhabited.
- OK.
How much of this stuff
comes to us every year?
Actually, huge amounts.
The Earth is growing by
at least 40,000 tonnes a year, so
a huge amount of material is falling
to Earth but we don't
really notice most of it
because the vast majority of
it comes in the form of dust.
Although several thousand meteorites
actually land on Earth every year,
most of those actually go unnoticed.
They fall just too far away from people.
If a meteorite falls maybe
15 feet away from you,
you probably won't notice it.
It will make a dull thud and that
will be it, unless it is very large.
This event is special
because it was so large.
There was no way you could not
notice this meteorite falling.
It really wanted to get noticed.
It said, "Ta-da! I am here."
Those events are spectacular
and they give us scientists these
important pieces of rock from which
we can learn about the solar system.
It is remarkable how we are able
to build up this picture of what
is going on millions of miles
away in the solar system.
It is one of the joys of science
really, almost like a detective
picking up on those tiny
clues to tell a bigger story.
So that the big question, the
one that really needs answering,
is why do some of these asteroids
suddenly head straight towards us?
Over 95% of asteroids are found
in an orbit between Jupiter
and Mars, called the main belt.
It's almost 200,000,000
kilometres across
and home to millions
of these orbiting rocks.
These asteroids have been following
the same path for millions of years.
So long as they remain here,
they pose no threat to Earth
but occasionally, one goes astray.
Collisions are one of the
reasons why this might happen.
But in the last decade,
we have learned that just a few
rays of light are enough, because one
scientist has tracked the orbit of
just one of these millions of rocks.
Steve Chesley of NASA's Jet
Propulsion Lab in California
has made a study of 200,000,000
tonne asteroid called Golevka.
This is a model of Golevka.
It is actually about 500 metres across,
say the size of a football stadium.
It rotates in this direction.
As you can see, it has a
very angular shape to it.
He set out to investigate a
100-year-old theory that said
asteroids were powered
by the sun itself.
It was called the Yarkovsky effect.
The Yarkovsky effect is a very small
acceleration and acts on asteroids
and what it is is,
if you take a model, the
sun is hitting the asteroid,
warming the surface, and
as the asteroid rotates,
that hot surface radiates
the heat out in a different
direction into space and that
causes an acceleration, a very slight
acceleration coming from the photons
that are emitted from the asteroid.
The idea is that this acceleration,
slight as it is, can have
significant effects upon
the orbits of asteroids
over millions of years.
It was an intriguing idea.
What sent asteroids out of
their orbit and on a path towards
Earth was photon propulsion,
but what was lacking was proof.
The Arecibo telescope is
over 300 metres in diameter.
It is one of the most powerful
telescopes in the world and it
uses radar to mark the precise
position of objects in deep space.
It was this telescope that would
allow Steve Chesley to detect any
tiny alterations in the
orbit of asteroid Golevka,
more than 15,000,000
kilometres out in space.
We knew that it would be in one place
if the Yarkovsky effect wasn't acting
on it, and it would be over here
if it was acting and
our models were correct.
When Steve and his team studied the
data, the results were unequivocal.
We knew from the radar measurements
where Golevka was within a few
tens of metres and yet it was
actually 12 or 15 kilometres
away from where it was predicted
to be without Yarkovsky effect,
so these very precise radar
observations allowed us to see
the 12-kilometre displacement
caused by the Yarkovsky effect.
So photons, those elementary
mass-less particles of light,
really can create a tiny force.
The force is about one ounce on earth,
say that the weight of
a shot glass, that is
the force on this huge asteroid,
the size of a football stadium.
Even for me it is truly remarkable,
it is dramatic that a force
so slight can have such dramatic
changes on individual asteroids'
orbit over millions of years.
The Yarkovsky effect is subtle.
It takes many millions of years
to gently nudge an asteroid
out of its regular orbit.
But once that orbit has been disturbed,
the consequences can be profound.
Now it can come increasingly under
the influence of the solar system's
largest planet
- Jupiter.
Jupiter has a mass 300
times bigger than Earth's
so there is a huge gravitational field.
Often that works to our benefit.
Stray objects can be swept
up in Jupiter's gravity,
drawing them into the planet.
We've actually observed Jupiter
acting as a shield in this way.
This photograph, from the
Hubble Space Telescope,
shows the fragments of a comet
torn apart by Jupiter's gravity
as the pieces were drawn
to the planet's atmosphere
the impacts left blast scars
- some as big as the Earth
..but there is a downside to Jupiter
it can also deflect asteroids into
orbits that cross the Earth's path.
The Chelyabinsk meteor appears
to be one of these typical
Earth-crossing events.
The likelihood is that it was
thrown out of its regular orbit
by either one or a combination
of the known causes -
collision, the Yarkovsky
effect, Jupiter's gravity.
It continued its new orbit
for hundreds, thousands,
even millions of years before
meeting its fateful end.
We can even begin to trace the exact
path that the Chelyabinsk meteor
took on its collision course with Earth.
Within the 16 seconds of action
are all the clues we need.
Now, from just one vantage
point it's not clear
exactly how far up it
is or how far away it is
but that's what we get from
looking at other vantage points.
So, here we are, again,
at a different angle.
The object is coming in,
almost out of the sun, there,
and by combining this video
clip with the other video clips,
what we can do is trigonometry.
Basically, you can figure
out how high up the object was
and how far away it was.
And if you catch the object early enough
then you actually know where
it was in the atmosphere
the first time you saw it.
So, you're kind of, triangulating
to get that fixed position
- and it changes over time so you get the trajectory?
- That's right.
In the first part of the
trajectory, what you've got there
is a path that is relatively
unaffected by the Earth's atmosphere.
So, we can use that part of
the video footage to track back
and figure out where this object
came from in the solar system.
I love watching this because I
now know where it is going to come
and you see it just hitting
the edge of the atmosphere.
It's going to bejust
Come on, come on
- There it is!
- Yup.
It's about 90 kilometres
up, at that stage,
travelling at 17.5
kilometres per second.
Using the different camera positions,
scientists have pinpointed
the exact position
at which the meteor
entered the atmosphere
and, by tracking the speed
and angle of the shadows
that the meteor casts,
they've calculated its velocity.
Together this is enough to
track back the asteroid's path
from deep space.
Although the asteroid and Earth
orbits are different durations
and at angles to one another
their clockwork regularity means
that we were bound to collide.
So, this shows, speeded up,
obviously, three and a half hours,
the last three and a half hours
of the life of this little rascal.
Yeah, it's nice to see it from
the asteroid's point of view.
The thing to remember is that this
asteroid has been in its orbit,
going around the sun, roughly
once every two years, we believe
- Minding its own business.
- Absolutely.
..and, unfortunately, on February
15 it found a planet in the way.
Sure enough,
at 09:20 hours the neat yet entered
our atmosphere above Siberia.
On this path and at time
it was Chelyabinsk that
took the full impact
..but could there have
been another scenario?
The meteorite landed at a
latitude of 55 degrees north,
had it arrived just a few hours later
we would have been
directly in its flight path.
So, was this a near miss for us?
If the asteroid had been in
a different part of its orbit,
so it didn't hit this
year but it hit next year,
it would have still
hit us on February 15th
but instead of coming in over Russia
it would have come in
over the UK and Ireland
and would have entered
the Earth's atmosphere,
in fact, entered the
North Atlantic Ocean.
In order for the meteorite to
strike anywhere near Britain
our paths through space would have
had to be fundamentally different.
So we know where asteroids come from
and the forces that shape
their date with destiny
but what exactly happens next?
The moment that a meteor strikes?
And what determines just how
devastating that strike will be?
When the Chelyabinsk meteor
reached our atmosphere
it was travelling at more than
65,000 kilometres per hour
and measured more than 15 metres across.
Apart from some unconfirmed reports
of craters at the
bottom of Lake Chebarkul
there's surprisingly
few signs of an impact.
Little of the 7,000 tonnes of space
rock that entered the atmosphere
have been recovered
..perhaps 300 fragments
..and yet, the effects were felt
over 3,000 square kilometres.
The question is how can apparently
so little do so much harm?
There's a clue from the last time
Earth experienced a meteor
strike on this scale.
On June 30th, 1908,
a huge explosion tore through
the forest of Tunguska, Siberia.
It was 20 years before the Russians
mounted an expedition to the site.
What they found astonished them
..60 million trees across
an area the size of London
had been levelled.
Scientists thought it has been
caused by a meteorite strike
..but then why was there no sign
of any kind of impact crater?
The answer is that the devastation
had to be caused by a meteor attack
of a very particular kind.
Physicist Mark Boslough
has been fascinated
by how so much destruction can be caused
without any apparent direct contact.
The explosion at Tunguska
was caused by an asteroid
that entered the atmosphere,
got close to the surface
and exploded before it hit the ground.
And that explosion created a blast
wave with hurricane force winds
that knocked trees over for
thousands of square miles.
Scientists call it an airburst -
a massive explosion in the atmosphere,
rather than on the ground.
As it enters the atmosphere at
speeds of up to 24 metres per second
the air resistance decelerates
the asteroid so fast
it breaks apart in a huge explosion.
Most of the damage from an explosion
like this is actually the blast wave,
it's the very high winds.
Mark created a simulation
to see what size
an asteroid would need to be to
generate such destructive power.
In this simulation I include more
of the physics to be more realistic.
We can see that the main shockwave
doesn't come out of the
point of the explosion
but it comes out of the point
where the fireball descends to.
So, by the time the
shockwave gets to the ground
it's much stronger than
it would otherwise be
and there's more damage on the ground
because the destructive
power was carried downward.
Based on Mark's calculations,
the devastation at Tunguska
could have been caused by an asteroid,
perhaps as small as 30 to
50 metres in diameter
..and this carries a
worrying implication.
Smaller asteroids are more
dangerous than we used to think
and because there are so
many more smaller asteroids
than bigger asteroids
we need to take that risk
more seriously than we used to.
The lesson of Tunguska helps
explain why in Chelyabinsk
there's so much damage but very
little meteorite to be found.
If we go back to the video footage
and we see the object coming in,
when it's in the high atmosphere
it suffers very little effect
but just here you
get this huge flare-up
and that's because the
atmosphere has become so dense
that it's almost impossible
for it to push through any more.
And, basically, something's got
to give, and the asteroid gives,
and it, basically, just breaks apart
in a huge catastrophic
fragmentation effect,
and that is what creates a shockwave,
which we hear as this sonic boom.
EXPLOSION
Really it's a balance between
the size of the object,
its speed into the
atmosphere and, critically,
the altitude at which it explodes.
Too high, if it's too small
and it explodes too high
the shockwave has little
effect on the ground.
If it'squite low in the
atmosphere, it's a large object,
then that shockwave is
completely devastating.
Actually seeing it in real
life really brings home to you
the energy that these things carry
and, even though it exploded tens of
kilometres, perhaps, up in the air,
so, quite a long way from the
ground, the force of the explosion,
the shockwave, was able to damage buildings
over a huge area and injure people,
and that was quite a
shocking thing to see.
The destructive power of
an air blast is immense
but, in a way, the people
of Chelyabinsk are lucky
because out there in the cosmos
is a different kind of asteroid,
one that poses an even greater threat.
I've seen the evidence of
what one of those can do,
the damage that it leaves behind,
and what you realise is the
Earth's own destructive forces -
you know, the great earthquakes,
the volcanic eruptions -
seem trivial in comparison.
This is Barringer Crater, Arizona
..the 50,000-year-old remnant
of a massive meteorite impact.
'This place really gives you a
sense of the destructive power'
of incoming meteorites.
The blast here would have
vaporised a city larger than London
but the lump of rock that did it
measured barely 15 metres across.
Down on the ground the scale of the
impact is even more breathtaking
..the crater is more
than a kilometre across
and nearly 200 metres deep.
The forces here were enormous,
the impact turned this solid rock
into this pulverised mush.
It justbursts out in your hand.
I mean, look at that.
They started out as
the same kind of rock.
The meteor that struck
here was about the same size
as the one that flattened Tunguska
but there is a critical difference
at Barringer the meteor didn't
explode in the atmosphere,
it struck ground.
So, this is just a fragment of the
true devastation unleashed here.
Fortunately, to understand
exactly why ground strikes
are so very destructive
we don't have to wait for
another Barringer to happen
because today we can
simulate this kind of impact.
And that's thanks to the
research of Pete Schultz
and one very special piece of equipment.
So, so, this was serial number one,
it was built during the Apollo time.
I guess because they thought
there would be several of them made
but this is the first
one and the last one.
And is the only one
like it, in the world.
This is NASA's Vertical Gun Range.
It was built to study how
impacts affected the moon
as the astronauts prepared to
make the first lunar landing.
We are armed, gated and reset.
Today, Professor Pete Schultz
uses it to model precisely
the dynamics of an asteroid impact.
We know that theseasteroid
impacts are bad
but you want to
understand really how bad.
Peter uses the NASA gun to fire
projectiles at very high speed
to simulate an asteroid
hitting the Earth.
So, for this experiment
we're going to fire
this tiny quarter-inch aluminium
sphere at very high speeds,
up to around five kilometres per second,
and then we will see what
type of crater it produces.
The target it will hit is made of sand.
So, we use sand because it records
the shock affects very clearly.
Outside of the impact chamber
are super high-speed cameras
that can film at up to
1,000,000 frames per second,
capturing every detail of
the impact and the aftermath.
- OK, lights out. Everything good?
- Yeah. - OK, we're out of here.
We have high voltage, the paddle
is in, the warning lights
androlling.
ALARM BUZZES
Oh, perfect. Perfect, perfect.
Now we're seeing the fireball come in
- it's brighter than the sun
and then, "Kapow!", it
hits the surface. Jeez!
This whole region, downrange,
would have been incinerated.
It would have been incinerated
just by this plasma,
this exploding vapour
plume engulfing everything.
There would have been winds that
would have been going so fast
it could pick up houses and spread
them hundreds of kilometres away.
This would have been Armageddon.
Experiments like this reveal
several important things.
One is that it's not just the impact,
it's all that vapour
that runs downrange.
In fact, you can see areas, here,
where there was so much
wind it actually carved out
pieces of this landscape.
So, what these experiments help us do,
they actually allow us
to witness the event -
see it in real time -
and try to understand the
processes that are going on.
It's really complex but we
have to see it to understand it.
So, asteroid impacts unleash a
trail of destruction far greater
than suggested by the
footprint of the crater alone.
Comparing the effects of an
airburst with a ground strike,
it seems the Chelyabins
got away lightly.
It's estimated that the
largest piece to hit the ground
weighed 500 kilos,
a fraction of the asteroid's
original mass of 7,000 tonnes.
Now if a piece of rock that
big had hit that area of Russia,
it would have produced
a huge impact crater.
Then that kinetic energy is
then delivered into the ground
and we see things like seismic shock.
So, you get People would
feel earthquakes on the ground.
So, the fact that it was an airburst
actually limited the consequences
for the people on the ground.
So, yes, still quite dramatic,
still, you know,
obviously, causing injuries
but it could have been a lot worse,
had it survived down to ground.
Ground strikes are amongst
the most destructive
natural hazards we know of.
When viewed from space,
Earth's encounters with giant
asteroids in its deep history
are revealed.
And there is evidence
from our planet's past
of a truly devastating meteorite strike
that decisively altered
the course of life on Earth.
Today, millions of
years after the impact,
the evidence for that
crater is well hidden.
SHE SHOUTS
This is a gateway to the cenotes,
the unique cave system of
Mexico's Yucatan Peninsula.
Wow!
Look at the size of this!
This is magnificent!
That is beautiful.
'This cave may be stunning,
'but it provides the evidence for
one of the greatest catastrophes
'in the Earth's history.'
And that water, it's so clear!
Lower the gear, please!
There's actually much
more to this amazing cavern
than first meets the eye.
But to understand the
scale of what happened here,
you have to go deeper still.
Underwater.
OK?
I'm not sure if I'm ready for this.
I've got all the equipment, but
there's something about
going down into water
when you're not quite
sure where your exit is
But I trust Bernadette completely here.
HE CHUCKLES
She knows what she's doing.
So I'm as ready as I'll ever be.
- Ready?
- All right.
Descending into the depths of the
cenote is like entering a new world.
Fewer people have visited
some of these drowned caverns
than the surface of the moon.
As divers have explored further,
they've discovered the cenotes
are actually part of a huge complex
of tunnels and caves.
In fact, when you look from above,
you can see there are cenotes
scattered across hundreds of kilometres.
And when they're mapped,
it becomes clear that they follow
a distinctive circular
course through the jungle.
They mark out the rim of a giant crater.
Scientific instruments show
the structure of the underlying
rock has been deformed,
revealing the boundaries of a
colossal meteorite impact crater.
This amazing cavern is part of a
bigger story, a much bigger story.
65 million years ago, THIS was the site
of one of the most catastrophic
impacts in Earth's history.
What became known as the
Chicxulub meteorite landed here.
And THAT triggered the
extinction of the dinosaurs.
The meteorite was 15 kilometres across,
enough to cause utter devastation
across the whole planet.
It exploded with a force of 100
million million tonnes of TNT.
The blast sent a giant plume of
vaporised rock out into space.
A crater was punched 30
kilometres into the Earth's crust.
It was above this rim of weakened
rock that these cenotes formed,
millions of years later.
The blast would have been ferocious.
But it was what happened next
that made the impact
a global catastrophe.
The blast plume that shot
into space fell back to Earth.
Billions of molten
particles superheated the air
to a temperature of hundreds of degrees.
Fires swept the planet,
choking the atmosphere
with soot and dust.
The dinosaurs, and most
other creatures, were doomed.
That discovery, back in the 1980s,
about what happened at Chicxulub,
changed everything.
Up until then, we thought
that the Earth had changed only
through grindingly slow processes,
but now we knew that there was
also sudden, violent catastrophes
that made the Earth the way it was.
Of course, what that meant
was that something like
this could happen again.
At any moment.
Luckily, the very biggest
asteroids are few and far between.
But there are still
plenty of rocks out there
that represent a
significant danger to us.
So, at the summit of an
extinct Hawaiian volcano,
Professor Nick Kaiser and his colleagues
are searching the skies
for killer asteroids.
Each night, using a revolutionary
billion-pixel sensor,
the team scans a vast swathe of the sky.
Follow me up to the next floor,
you'll see a better view
of the telescope itself.
They are looking for
any unidentified objects
that could be heading our way.
By capturing several images
of the same patch of sky,
separated by several minutes,
the team can see if anything's changed
against the background of stars.
You can see that there's a
dark thing and a white thing.
What that means is,
in these two exposures,
there was an asteroid, which
was here in the first exposure
and there in the second one.
It's kind of cute, here's
another one in the same image.
And, in fact, we'll detect hundreds
of asteroids in a single exposure.
Their observations are collated
at the nerve centre
of asteroid detection -
the Minor Planet Centre,
just outside Boston.
Its director is Tim Spahr.
And his job is to keep track of
every asteroid in the solar system.
Tim has developed a map to
visualise their location.
And, on that map, the most important
are the Near-Earth Asteroids,
the ones closest to the planet.
On the screen here is a
map of the solar system.
And I've got the sun in the centre
and the third planet out here
would be that of the Earth.
The red dots in here are
actually Near-Earth Asteroids,
the green ones are the
regular Main-Belt Asteroids.
There are over 9,000
Near-Earth Asteroids.
But there's one type they're
particularly concerned to locate
..those asteroids that are
over one kilometre in diameter.
An Earth impact with one of
these would spell disaster.
Tim's data reveals
that there are 900 asteroids
bigger than a kilometre
in those dangerous near-Earth orbits.
But he has some good news.
Right now, there's no information
that any of those large objects will
hit the Earth in the next 100 years,
so we're safe from impact of those
objects for at least 100 years.
So there are no catastrophic
asteroid impacts on the horizon.
But there are still dangers out there.
On 6th October 2008,
asteroid hunter Richard Kowalski
saw something that would change
the assessment of threats
presented by asteroid impacts.
The night was proceeding normally
and up on the screen
came another asteroid.
As I continued to make
observations throughout the night,
it appeared to be
moving slightly faster.
And this indicates that the
object is close to the Earth.
As with any other asteroid,
Richard reported what he'd
found to the Minor Planet Centre.
I got up in the morning
about seven o'clock
and I had a message
on the computer saying,
"Could not compute an orbit
for a particular object."
I grabbed the observations of
this object and I computed an orbit
and it was immediately
apparent, right then,
that that object was
going to hit the Earth.
And, sort of ominous fashion,
it said it was in 19 hours.
Following a strict written protocol,
Tim quickly reported the findings to
NASA's asteroid investigation team,
in California.
We got a call from Tim Spahr,
at the Minor Planet Centre,
saying we had an impacter
coming in, in less than 24 hours.
That woke me up.
NASA's expert on asteroid
orbits, Dr Steve Chesley,
raced to verify the data.
The first thing I saw was a 1.000,
a 100% probability of impact
in less than a day's time.
I'd never seen anything like this
outside of simulations
and software testing.
An asteroid strike would
create a huge explosion.
NASA feared this might even
be mistaken for a nuclear bomb.
We wanted folks to know
this was a natural event,
by Mother Nature rather
than some sort of man-made
event like a missile
or something dreadful.
Information passed rapidly
up the chain of command.
NASA headquarters notified the
White House that this was coming.
Everyone wanted to know
where it would strike.
NASA predicted a remote
area of the Nubian Desert.
At 2:45 in the morning,
NASA were proved right.
The explosion created a vast
fireball burning as hot as the sun.
It was so big and so hot this image
was captured by a weather satellite.
And yet the object that caused
it was only four metres across.
Smaller than the asteroid
which exploded over Chelyabinsk.
I definitely think the
impact was a wake-up call.
I have to admit I never thought
I'd see that in my career,
where we would discover something and
it would hit the Earth later that day.
What was worrying about that
impact was that the asteroid was too
small to detect until it was
very, very close to the Earth.
Of course, for Chebarkul, it wasn't
even spotted until it was already here.
But we are getting better at
spotting smaller asteroids.
On the same day that Chebarkul
was hit, another asteroid,
similar in size to the object
that created the Barringer Crater,
came within just 28,000
kilometres of the Earth.
Approaching from beneath the
planet, asteroid 2012 DA14,
passed inside the orbit of
our geostationary satellites
before heading off to the north.
This asteroid had been
successfully tracked for a year.
Despite its proximity, scientists
knew that it posed a threat.
So we know we are safe for at least 100 years from
most near-Earth asteroids over a kilometre in size.
We are better at detecting
objects down to 50 metres across,
like DA14.
But for asteroids smaller than that,
like the one which exploded over
Chelyabinsk, we still
have little or no warning.
There are still some we haven't found.
So there's this unknown bit of the
equation where we are still looking
for some, we know they are there
but we don't know where they are.
So this is a threat, but
hopefully as technology moves on,
we'll always have a much better idea whether
one's going to pose a risk to the Earth.
We could see an event tomorrow
or in 10 or 20 years time,
that we hadn't previously detected.
That is always the risk we face.
Until we can catalogue and
identify all the hazardous
objects in the solar system,
that risk will always remain.
And there's one other factor that can make it
particularly hard to spot an incoming object.
It's the reason why no-one saw the asteroid
that was hurtling towards Chelyabinsk.
It came in in the daytime
sky out of the sun.
Right.
We've got telescopes looking
out there for these objects,
but they only work at night.
Radar doesn't help either,
because to really use radar,
to find these objects, you have
to know exactly where to look.
If you don't know what's coming
in, you don't know where to look.
Because of that then, this
thing and objects like this,
if they come in at that particular
direction they're always going
to take us by surprise at the moment
with our current survey system.
But even if we can spot an
asteroid heading towards us
and in good time to prepare,
what if anything can we do?
There's different options for deflecting
asteroids and it is a bit sci-fi at the moment.
The idea of shooting it out of
the sky with a nuclear weapon
would really be a dreadful idea.
It would just shower us
with radioactive debris,
and it would just be
do more harm than good.
What would be much better
would be to push it, nudge it
slightly off its course so that
it wasn't then going to collide.
So how do you gently nudge an asteroid?
There's lots of different
techniques to push it.
So The one I love
is called a mass driver.
There's a machine, which sits on
the asteroid and throws off rocks,
so it is accelerating rocks that way
and that makes the asteroid gradually
move in the opposite direction.
You can paint one side
of the asteroid white.
That reflects the sun and there's this weird effect
that makes the asteroid gradually drift across.
We can launch a mission now, which
is essentially can impact an asteroid
and then deflect it
- a bit like a billiard shot or a snooker shot.
We just hit the asteroid
extremely fast with a spacecraft
and that small impact is
sufficient to just alter its course
so that it misses the Earth.
When you consider Earth's history,
stretching over billions of years,
it's clear that meteorite
impacts, far from being unexpected,
are just a normal part of
the life cycle of our planet.
But that is not how they seem to us.
The Chebarkul meteorite
is a reminder of something
we would probably rather
not think about too often -
how a sudden, apparently random event
could have devastating consequences.
EXPLOSION AND SCREAMING
But this time we have been lucky.
Although it was terrifying
for those who witnessed it,
this meteor struck without
causing any fatalities.
And close enough to be
captured on multiple cameras.
So it's given us a huge amount
of information to help us
prepare for the next one.
I think perhaps the real lasting
legacy of the Russian meteor
will be the effect it has had on the popular
consciousness and perhaps on politicians.
Scientists have been saying for decades now
that these things do happen from time to time,
that they could be dangerous if
they happened over populated area.
But now we have actual proof,
we have an event we can point to.
We know it could've
been worse than this.
So, I think if this leads to
more vigilance and perhaps,
the detection of future impacting
events, that'll be a good outcome.
When a bit of an asteroid, comes
through the atmosphere and lands on
the Earth as a meteorite, it reminds us
that the solar system is a dynamic place.
It's It's not finished.
It's still working. It's still
evolving and still changing.
So next time you look up at
the night sky, spare a thought
for those thousands of rocky
lumps whizzing across our path.
A few of them have got our name on them,
but the thing is by analysing in
detail the data from the meteor,
it means that next time, and
there will be a next time,
we will be much better prepared.